At issue is a bill before the Illinois legislature that would raise the mandatory minimum sentence for people caught illegally possessing a firearm. The proposal would push the minimum up from the current one year, with a requirement that they serve at least 50 percent of their sentence, to three years, with a requirement that offenders serve at least 85 percent of their sentence. This new law is needed, says University of Chicago Crime Lab Director Jens Ludwig, because current sentencing practices aren't deterring crime. For instance, Ludwig writes in a recent memo [PDF] that out of all the people in Illinois who are charged with illegally carrying a firearm and then put on probation, "over 63% of these probationers are re-arrested for some crime within 12 months, with 7% re-arrested for a violent crime specifically."

Here's how complicated the divide over this issue has become: Chicago Mayor Rahm Emanuel, Chicago Police Department Superintendent Garry McCarthy, and Cook County State’s Attorney Anita Alvarez all support the bill. The NRA, the John Howard Association of Illinois (which "promotes public safety through cost-effective prison reform"), Families Against Mandatory Minimums, and University of California Berkeley Law Professor Franklin Zimring (who is also the former director of U. Chicago's criminal justice program) oppose it.

Ultimately, this split among advocates and scholars is about the effectiveness of mandatory minimum sentences.

In theory, mandatory minimum sentencing policies are designed to ensure sentences are determined by an objective calculus. The idea being, certain offenses should require a certain sentence, every time, regardless of mitigating factors like how old (or young) the offender is, his or her gender, whether it's that person's first offense, or if the person is a breadwinner or parent. In theory, knowing that a certain crime will be met with a consistently harsh sentence will deter would-be criminals from that kind of crime. In Chicago's case, we're talking about carrying a firearm.

That's Ludwig's argument in a nutshell. Conversely, in his memo, Ludwig writes that inconsistent sentences are one reason why gun crime in Cook County is so problematic:

One challenge in at least some parts of Illinois at present, such as Cook County, is the variability or inconsistency in punishment for [unlawful use of a weapon] cases within the court system. A 2011 analysis by the Chicago Sun-Times of sentences handed down for gun possession in Cook County found that while nearly 75% of defendants were sent to prison, 14% received probation, 6% received boot camp, and 4% community service (Main 2013). A Chicago Tribune analysis of people charged with UUW near Cornell Square Park between 2008 and 2012 found that 60% received probation (Sweeney, 10/1/13). Any measure capable of reducing this variability and increasing the consistency or certainty of sanctions would be expected to help reduce gun carrying through deterrence.

Rebutting Ludwig is Franklin Zimring, who writes in the ChicagoSun-Times that the Crime Lab economist has things all wrong. Zimring points out that harsh sentences already exist in Illinois for illegally carrying a gun (though they are not mandatory), and that mandatory minimum sentencing policies do not prevent criminals from getting lighter sentences, they just transfer the privilege of discretion from judges to prosecutors.

On that last point: Five years after New York state increased the mandatory minimum sentence for illegally carrying a gun (from one year to more than three), The New York Timesreported that "fewer than half the defendants who had been arrested for illegal possession of a loaded gun in New York City received a state prison sentence."

Why? The Times again:

"Sometimes...prosecutors reduce the charges appropriately as a result of plea bargaining, which spares the time and expense of going to trial in a marginal case, as when a weapon is found in a car carrying several people and it is difficult to prove who had physical possession, or the seizure is subject to constitutional challenges over its reasonableness."

As Zimring notes, such conduct is also common among Cook County prosecutors, "who allow and agree to plea bargaining in the vast majority of felony convictions for carrying guns. If these results are what prosecutors consider appropriate, why don’t the prosecutors oppose mandatory minimums?"

Ludwig comes close to acknowledging the existence of prosecutorial discretion, but doesn't actually name the culprit. I suppose it's possible that he's simply not aware of how prosecutors do their jobs. More likely is that he is aware, and knows that increasing the mandatory minimum from one year to three could increase the sentence a plea bargainer would be willing to take from probation to jail time.

But let's put aside the motivations of all involved. To really test out the deterrent powers of mandatory minimums, you'd have to forbid plea bargains. Every single case would have to go before a jury, (or, if there's not enough evidence, be dismissed). Every single guilty offender—regardless of age, sex, or other mitigating factors—would have to be sentenced to the mandatory minimum. You'd have to tell juries this fact before they deliberated.

Prosecutors would howl as their conviction rates slowed, pretrial detention levels would balloon, and court costs would explode, but we'd at least have better data on whether mandatory minimum sentencing works.